The present invention relates to a mobile robot and a control method therefor. More particularly, it relates to a mobile robot to operate in an environment shared by human beings and a control method therefor.
For a robot to operate in an environment shared by human beings, it is ideal for the robot to be as tall as human beings so as to efficiently carry out manual operation to assist human workers or operate interactively with them. In a case in which a robot is required to efficiently carry out operation which involves traveling in a space where human beings are also present, it is desirable to minimize the footprint (flat projected area) of the robot. When such requirements for the shape of a robot are met, the center of gravity of the robot inevitably becomes higher relative to the footprint and the robot becomes less stable in its standing position. On the other hand, when a robot is to be put in practical use, it is required to be capable of traveling quickly and safely. This requirement is not compatible with the higher center of gravity of a robot described above.
In view of the requirement, robots provided with a coaxial two-wheeled mechanism and using inverted pendulum control to control a forward thrust have been proposed as disclosed in, for example, Japanese Patent Laid-Open No. S63 (1988)-305082, Japanese Patent Laid-Open No. 2003-271243, and Japanese Patent Laid-Open No. 2004-74814.
The patent document 1 describes a method for controlling the attitude of a coaxial two-wheeled vehicle having a vehicle body rotatably supported on an axle which is, at its ends, provided with a pair of wheels, a wheel driving motor attached to the vehicle body, a control computer to send operation commands to the wheel driving motor, and an angle detecting means for detecting an inclination of the vehicle body. In the method: the angle detection means detects the angle of inclination of the vehicle body; the angle of inclination of the vehicle body detected is sampled at short intervals; a controlling torque of the wheel driving motor is determined based on calculation made by substituting the sampled inclination angle as a state variable and a feedback gain as a coefficient into a control input calculation formula preset in the control computer; and the control computer sends a command for operation equivalent to the control torque thus determined to the wheel driving motor to control the attitude of the coaxial two-wheeled vehicle.
The Japanese Patent Laid-Open No. 2003-271243 discloses a communication robot including an upper body mechanism which has plural axles and which is installed on a carriage. The communication robot is also provided with substantially one axle, two wheels attached to the axle to be apart from each other, and a control means which controls the rotation of the wheels according to the state of the upper body mechanism.
The Japanese Patent Laid-Open No. 2004-74814 discloses a human-carrying vehicle which has left and right coaxial drive wheels and which performs, to keep itself balanced in the front-back direction, attitude control and travel control by controlling the drive wheels according to the output of an attitude sensor. The vehicle is provided with an auxiliary wheel which contacts the ground in front of and/or at the rear of the driving wheels and an auxiliary wheel driving section which puts the auxiliary wheel out and in.
These vehicles use a coaxial two-wheeled mechanism with a small footprint. The coaxial two-wheeled mechanisms of these vehicles have a dynamic characteristic of a statically unstable inverted pendulum and are kept stably erected by dynamic attitude control. They can travel in a stable state making quick acceleration and deceleration by moving their gravity center.
Even though the conventional techniques disclosed in the Japanese Patent Laid-Open No. S63 (1988)-305082 and Japanese Patent Laid-Open No. 2003-271243 can improve the stability of a robot traveling forward or backward, they lack consideration for the stability of a robot which travels quickly making sharp turns. In other words, they include no measures against such problems as falling to a side on account of a centrifugal force generated when making a sharp turn, or slipping of the wheels on account of unbalanced ground reaction forces acting on the wheels and resultant failure of inverted pendulum control causing the robot to fall.
The conventional technique disclosed in the Japanese Patent Laid-Open No. 2004-74814 is for a two-wheeled mobile mechanism used as a vehicle to move a person. The vehicle is provided with a saddle to support a rider and a handlebar mechanism for the rider to hold. The handlebar mechanism has a passive degree of freedom to swing laterally. Such freedom to swing makes it easier for the rider of the vehicle to use, for example, when one of the wheels of the vehicle runs on an obstacle, his or her reflexes to keep his or her attitude stable in the lateral direction. Even though the technique disclosed in the Japanese Patent Laid-Open No. 2004-74814 makes up a means for stabilizing the vehicle attitude in the lateral direction, it is dependent on the rider's reflex movements. Therefore the technique cannot be said to provide a means for stabilizing and controlling the attitude in the lateral direction of an autonomous mobile robot which makes a two-wheeled travel using inverted pendulum control.